The present invention relates to a gas injection system for injecting a refrigerant gas into the compression process of a screw compressor of a refrigeration cycle for the purpose of eliminating reduction in the refrigeration power, and more particularly in a so-called economizer cycle which employs a subcooling device for subcooling a liquid refrigerant.
A known refrigeration system employing the above-mentioned economizer cycle incorporates a screw compressor having a capacity controlling or unloading means constituted by a slide valve. In such a refrigeration system, the refrigerant gas which has subcooled the refrigerant liquid in the subcooling device is injected into the compression chamber of a screw compressor in the compression phase so as to avoid any reduction in the refrigeration power.
Such a gas injection system for injecting refrigerant gas into a screw compressor is proposed in, for example, U.S. Pat. No. 4,005,949. This proposed gas injection system, however, suffers from a disadvantage in that the injected refrigerant gas is undesirably introduced into the suction side of the screw compressor, if the gas injection is performed when the screw compressor is operating in the unloaded state with its slide valve opened thereby adversely affecting the merit of the refrigerant gas injection and causing various unfavourable effects.